I'm wondering if a notarized letter of permission from an absent parent is needed if you have a birth certificate and passport for the child?

My husband and I have custody of our step-son, age 17 and his mother is not being cooperative in sending a notarized letter of permission. 9We've requested several times with no answer) He has the same last name as us, and his application for a passport went through and we do have that in hand as well as his birth certificate.

This could be confusing, but I'd say if he has a passport with your last name it shouldn't be a problem. Have you looked at the requirements for the cruise line, and perhaps at the State Department website?

I've checked the passport website and just read pre-passport instructions but I don't find anything listing travel requirements once a passport is in hand. Carnival's instructions strongly recommend a notarized, signed letter of permission from the other parent.

I'm leaning on the side of there should be no problems since my husband is the custodial parent and not needing her signature for his approved passport application in the first place....but I don't want to chance ruining our vacation either...........

That can be a very sticky thing when going out of country. Our sons has to leave permission for his wife (they are married) to leave the county. He is in the Navy and usually has a check list, she travel with the kids to Japan when he ships out.
An old room mate of mine had sole custody og his daughter, but he could not take his daughter to Canada without his x"s permission. My X brother in law got in trouble to taking his kids out of the COUNTY. A mad X can make thing very very bad. I would get your ducks in a row before trying to take them out of country.

Then why does customs required our daughter-in-law have a notarized letter from our son? Just because they have a passport it does not mean permission by both parents was given. It kids one parent from kidnapping the kids and moving out of county. It is the law. When she travels to Japan or even Canada she has to have the letter... it is checked.
My Cousin had a had time getting his son into Canada, she wife is dead. They talked to the son and let them pass. He now carries a copy of the death cert. with him.
Again you should get your ducks in a row before trying to take them out of the Country!!